Optical fibers have been used in many types of sensing devices to detect specific media, phase changes in media, and media pressure. For example, the medical field uses fiber optics to measure the intravascular blood pressure of patients. The automotive industry employs fiber optic pressure sensors to measure pressure inside internal engine combustion chambers. In addition, the oil industry makes use of fiber optics to analyze the contents and pressure of oil and gas tanks, as well as oil wells.
Typically, such devices consist of a light source, such as a laser, and a light detector, such as a charge coupled device, connected by an optical fiber. Light from the light source is continuously transmitted through the fiber to the sensor. At the sensor, the light is affected by the media, resulting in a change in light intensity or power. The light then travels to the light detector, where the change in intensity or power is interpreted. The information is then displayed to the user.
Usually, an optical fiber consists of a core and cladding surrounded by a jacket. The jacket functions as a protective barrier, as it does not permit light from the core of the fiber to pass into the environment nor light from the environment to pass into the core. Thus, a jacket ensures that an optical fiber will have almost complete total internal reflection, with minimal light loss due to transmission through the jacket.
How the light is affected at the sensor depends on how the optical fiber is used. Optical fibers can be used extrinsically or intrinsically. Intrinsic fibers are active components of the sensor as their main characteristic, total internal reflection, is altered.
For intrinsic fibers, a portion of the jacket is stripped away, allowing a small amount of light transmission through the cladding or core. The resulting decrease in light intensity or power can be measured by finding the difference between the intensity of the light emitted by the light source and the intensity of the light received at the light detector. The amount of light lost corresponds to a change in the environment, such as a change in media or a change in phase of the same media. An intrinsic fiber sensor may be placed between two normally jacketed optical fibers, or it may be placed at the end of a normally jacketed optical fiber, depending on the user's needs.
Extrinsic fibers are used to detect changes in environmental pressure. They comprise unaltered optical fibers that have a membrane and a reflecting device, such as a mirror, at one end. The fiber is typically positioned vertically, with the membrane and mirror located at the bottom end. Light from the light source travels down the length of the fiber until it encounters the mirror, where it is reflected. The jacket is not removed from any part of the fiber, so total internal reflection is maintained. The sensor is usually lowered into the environment to be analyzed. At increasing depth, the upward pressure increases and the membrane is deformed. The change in position of the membrane causes a change in the position of the mirror. When light from the light source travels down the optical fiber, it will hit the mirror and be reflected back at an angle corresponding to the deformation of the membrane and hence the pressure. The change in reflection angle results in scattering of light and thus a variation in the intensity of the light that is reflected back to the light detector. Using information about the pressure of a medium allows the user to calculate the depth. If the dimensions of the container holding the medium are known, the volume can easily be calculated.
U.S. Pat. No. 5,005,005 by Brossia et al. discloses a sensor to detect environmental media and pressure which makes use of two intrinsic optical fibers. One sensor comprises an unjacketed part of optical fiber between two jacketed parts. This sensor is capable of distinguishing between phases, such as water and ice, and different substances, such as water and gasoline. The second sensor is similar to the first except that it contains a number of unjacketed fiber optic segments and it is placed vertically in the substance to be analyzed. This sensor is capable of distinguishing volume, as the depth and change of depth of the substance is sensed by the number and position of sensors in contact with the substance.
U.S. Pat. No. 5,362,971 by McMahon et al. discloses a fiber optic detection system with an intrinsic fiber sensor in a U-shaped configuration. With such a fiber optic loop, the amount of light lost can be determined by the degree of curvature of the loop. The diameter of this loop can be predetermined to provide optimum results for each substance to be analyzed. In addition, the loop improves compactness.
U.S. Pat. No. 5,606,170 by Saaski et al. discloses a multifunctional sensor system which uses a number of optical fibers to detect environmental media. Of interest is the use of numerous fibers in different patterns to transmit light to and from the sensor.
U.S. Pat. No. 5,385,053 by Wlodarczyk et al. describes a fiber optic pressure system which makes use of an external sensor intended for use in the automotive industry. In this invention, the deformable membrane is less sensitive to higher temperatures, which can distort pressure readings.
Finally, U.S. Pat. No. 5,425,273 by Chevalier discloses a fiber optic pressure system with inclusions in a compressible transparent material. The pressure sensors comprise optical fibers surrounded by a material which is essentially elastic to deformation and is optically transparent to the wavelengths of light used. At least two sensors are used. The distance between the sensors varies when the length between them is deformed. This, in turn, modifies the focal length of the defined optical system and leads to a variation in the light energy collected by a light detector.
None of the above inventions combine the two types of sensors, intrinsic and extrinsic, to design a detector which has the ability to detect both environmental media and pressure. U.S. Pat. No. 5,005,005 can detect the relative level of a fluid in a container, which can be used with other measurements to determine volume; however, it cannot detect the pressure of a fluid.
In addition, no mention is made of protecting the fragile optical fibers which are exposed. The sensors are often lowered to great depths into tanks and in the process the fibers frequently hit against the tank walls. In the case of the oil industry, which produces highly flammable products, the sensing devices are thus vulnerable to explosions. Over and above such major hazards, minor damage to the fibers can result in changes in light reflected or transmitted, which leads to faulty readings.